1. Field of the Invention
The present invention relates in general to audio data processing and in particular, circuits for controlling a bidirectional terminal and systems using the same.
2. Description of the Related Art
The ability to process audio information has become increasingly important in the personal computer (PC) environment. Among other things, audio is important in many multimedia applications, such as gaming and telecommunications. Audio functionality is therefore typically available on most conventional PCs, either in the form of an add-on audio board or as a standard feature provided on the motherboard itself. In fact, PC users increasingly expect not only audio functionality but high quality sound capability. Additionally, digital audio plays a significant role outside the traditional PC realm, such as in compact disk players, VCRs and televisions. As the audio technology progresses, digital applications are increasingly sophisticated as improvements in sound quality and sound effects are sought.
One of the key components in many digital audio information processing systems is the decoder. Generally, the decoder receives data in a compressed form and converts that data into a decompressed digital form. The decompressed digital data is then passed on for further processing, such as filtering, expansion or mixing, conversion into analog form, and eventually conversion into audible tones. In other words the decoder must provide the proper hardware and software interfaces to communicate with the possible compressed (and decompressed) data sources, as well as the destination digital and/or audio devices. Since, there are a number of different audio compression/decompression formats and interface definitions, such as Dolby AC-3 and S/PDIF (Sony/Phillips Digital Interface), a state of the art digital audio decoder should at least be capable of supporting multiple compression/decompression formats. In addition, the decoder must have the proper interfaces required for overall control and debugging by a host microprocessor or microcontroller.
During the decompression of streaming compressed audio data, as well as during other decoder operations, arithmetic and logic operations must be continuously performed. In order to maximize overall system efficiency and data throughput, the speed at which the arithmetic and logic operations are performed also must be maximized. Thus, the need has arisen for methods for increasing the speed of arithmetic and logic operations in high speed applications, including audio decoders.
According to one embodiment of the principles of the present invention, circuitry is disclosed for controlling a bidirectional terminal. An output transistor is provided for selectively coupling the bidirectional terminal to a voltage rail, the output transistor turning on when a voltage at a control node falls below a preselected threshold voltage. A diode is coupled to the control node which has a threshold voltage lower than the threshold voltage of the output transistor and maintains the output transistor in a substantially turned off state by maintaining the voltage at the control node above VDDxe2x88x92VT, of the output transistor, where VDD is the supply voltage and VT is the threshold voltage of the output transistor.
By advantageous use of the short channel effect, the inventive concepts insure that the output transistor fully turns off during a transition of the pad from a transmit node to a receive node. In particular, these concepts insure that the output impedance rapidly transitions to a high state when the pad is transition from a logic high output node to a high impedance input node. Moreover, these concepts can be applied to systems which are tolerant to received peak signal voltages in excess of the on chip power supply voltage. Additionally, the inventive concepts also insure that the turn-off of the output transistor occurs independent of process variation.